1. Technical Field
The present invention relates to a toothed belt that has high stiffness, excellent high load resistance, and environment resistance such as heat resistance and cold resistance, and achieves high accuracy, compactness and low noise.
2. Description of Related Art
The toothed belt is wound between a driving toothed pulley and a driven toothed pulley, and is used, for example, as a power transmission belt for general industrial apparatuses and OA apparatuses, a timing belt for vehicle internal combustion engines, and a driving belt for bicycles. The toothed belt is normally constituted by a belt body formed of a black rubber portion containing carbon black embedded with a plurality of core wires extending in the direction of the length, a plurality of tooth rubber portions formed on a surface of the belt body, and tooth cloth portion covering the surface of the tooth rubber portions. The toothed belt becomes unusable in occasions such as when tooth breakage occurs mainly because of tooth cloth wear or when a core wire breaks.
To improve the transmission capacity, the stopping accuracy and the attenuation characteristic of the toothed belt, it is essential to enhance the strength, the stiffness, the wear resistance, the bending fatigue resistance and the adhesiveness, to other materials, of the rubber portion. Japanese Patent No. 4,360,993 discloses an invention of a toothed belt where the strength, the stiffness and the wear resistance of the rubber portion are enhanced by blending, as a constituent of the rubber portion, a polymer alloy in which zinc methacrylate is finely dispersed in hydrogenated nitrile rubber (HNBR).
Moreover, to improve the transmission capacity of the toothed belt, it is essential to improve the wear resistance and the adhesiveness, to the rubber portion and the core wires, of the canvas for the belt tooth cloth.
Further, to improve the transmission capacity, the stopping accuracy and the attenuation characteristic of the toothed belt, it is also essential to enhance the strength, the stiffness, the bending fatigue resistance and the adhesiveness, to other materials, of the core wires.
Japanese Patent No. 5,116,791 discloses an invention of a toothed belt having high stiffness and high strength and further, having excellent bending fatigue resistance since the rubber portion contains a hydrogenated nitrile rubber the Mooney viscosity (1+4) 100 deg C. of which is not less than 100 and not more than 160.
The above-described toothed belts of Japanese Patent No. 4,360,993 and Japanese Patent No. 5,116,791 are mainly for general industry, and are unsuited for use as a toothed belt for uses in vehicles which is used, for example, in an engine room, is a small pitch toothed belt having a tooth pitch of not more than 3 mm, and requires environment resistance, high load resistance (compactness), high accuracy, light weight and low noise.
For a small pitch toothed belt, as a rubber constituent of the rubber portion, CR (chloroprene), EPDM (ethylene-propylene-diene rubber) or the like is frequently used, and for use under low load, the rubber hardness Hs of the rubber portion is 72 to 83 and the 100% modulus at a vulcanized rubber test is 3 to 6 MPa, which indicates that stiffness is insufficient for achieving compactness.
As for the service temperature of the small pitch toothed belt, the upper limit is normally 80 deg C., in particular, 120 deg C. in the case of EPDM, and the lower limit is normally −15 to 20 deg C., in particular, −40 deg C. in the case of EPDM. In the conventional small pitch toothed belt, when heat resistance and oil resistance are satisfied, cold resistance is insufficient, when cold resistance is satisfied, oil resistance or heat resistance is insufficient, and when water resistance is satisfied, durability and dimension stability are insufficient. As for environment resistance, although belts tailored to a specific required physical property have conventionally been present, properties other than the specific property are apt to be low. Particularly, in the case of EPDM, oil resistance is insufficient. In the case of the EPS (electric power steering), scattering of grease from a ball screw occurs.
When the rubber portion contains CR and EPDM, from the viewpoint of strength, workability and moldability, it is difficult to realize high stiffness which leads to compactness.
In the conventional small pitch toothed belt, bending fatigue resistance and wear resistance are insufficient, permanent deformation is large, cracks are readily caused in the back rubber and the tooth portions, and tooth breakage readily occurs.
Moreover, in the conventional small pitch toothed belt, since glass core wires having a small diameter and a low modulus of elasticity are used, rubber stiffness is low and this increases the belt width, so that low noise is difficult to achieve. For example, in the case of a timing belt for the EPS, although the tooth pitch is 2 to 3 mm with the intension of achieving low noise, since the belt width is 25 to 34 mm, considerable space is required, and noise is caused as a result. When helical teeth are adopted, although low noise is achieved, the side surfaces of the belt are shaved because of a significant reduction in dynamic tooth jumping torque and an increase in side thrust, and durability is significantly low because of the occurrence of uneven wear of the tooth cloth or the like.
As for the original yarn of the core wires, since a thin core wire made of E glass is mainly used, strength, bending fatigue resistance and stiffness are insufficient, so that it is difficult to achieve high compactness. Although use of a high strength glass is considered, the increase in strength is 30% at the most. In addition, since an RFL treatment is performed, permanent elongation, heat resistance and water resistance are insufficient, and environment resistance is insufficient.
In the case of the small pitch toothed belt, since the tooth portions are small, the original canvas for tooth cloth is thin, so that strength is insufficient (particularly, the woof). When strength is ensured by using thick cloth, moldability is poor (the belt tooth portions are mostly cloth), so that durability is insufficient.
When the RFL treatment or a mucilage treatment using a mucilage containing carbon is performed as the treatment of the tooth cloth, friction is high, and the surface is shaved by scattered carbon shreds to reduce wear resistance. The RFL treatment is a treatment performed by using an RFL liquid formed by causing a condensation reaction between resorcinol and formaldehyde under alkaline environment and adding formalin latex or the like thereto to cause a reaction.
When the tooth portions of the belt are covered with the tooth cloth, it is preferable that the junction (joint, seam) be at a tooth tip; however, when the tooth pitch is not more than 3 mm, since the teeth are too small, it is difficult to situate the junction of the cloth at a tooth tip. The junction is shifted by the degree of linearity of the junction of the tooth cloth, a twist caused when the core wires are wound, and the like. Therefore, currently, a sewing machine and welding are used for joining, and formation into a belt is performed without specific consideration given to the position of the seam (this hardly causes a problem since the belt is used under low load). However, in the case of joining by a sewing machine, the back rubber sometimes oozes out to the tooth side surfaces through the needle holes to cause an unusual noise, and in the case of welding, a molding defect (the welded junction is entangled, so that the entire surface of the belt tooth is cloth) sometimes occurs at the junction.